The present invention relates to a semiconductor device having a circuit which includes at least one transistor and, more particularly, to a semiconductor device having a transistor circuit of a high withstand voltage which includes an overvoltage protection diode with a controlled breakdown voltage.
In a high power transistor circuit, a combination of overvoltage protection diodes are used to protect the transistor elements. These protection diodes must be connected to the collector-base paths of the transistors on one chip. Furthermore, a breakdown voltage V.sub.Z of the protection diode must be lower than a DC collector-to-emitter voltage (base open) V.sub.CEO.
Such a protection diode is connected to the collector-base path of the transistor on one chip in the following manner. An impurity is deeply diffused from part of an inactive base region to a collector region of a high concentration, thereby determining the breakdown voltage V.sub.Z. Alternatively, an impurity is ion-implanted in part of the inactive base region to form a high-impurity region of the same conductivity type as that of the collector region, thus determining the breakdown voltage V.sub.Z.
However, the protection diode of the type described above must be formed in a separate process from a process for forming a general transistor. Furthermore, in the case of an ignitor output circuit for an ignition control circuit of an internal combustion engine, the breakdown voltage of the protection diode must fall within a range of 360 V .+-.10%. it is very difficult to set the breakdown voltage in the above range.